RKPv2 spec clarifications

- Rename keysToCertify to keysToSign to match param name in a few
  places.
- Make clearer that P-256 is allowed in EEK.
- Make clearer that EEK curve should match that specified in
  RpcHardwareInfo.supportedEekCurve.
- Add note about AAD for AES-GCM encryption.
- Add note about format of public key data in KDF context.

Test: None, comments only
Change-Id: Ie775a453b4d3941b7d24396da66681ab91978d81